The ordinary method of producing a titanium strip coil is explained in the following. The method starts with a large ingot obtained by melting using the consumable electrode arc melting method or electron beam melting method and solidification. In the case of the consumable electrode arc melting method, the shape of this large ingot is a cylinder of about 1 meter diameter, while in the case of the electron beam melting method a rectangular shape is also produced that has a cross-section of about 0.5 to 1 m per side. Since the cross-section is so large, the large ingot is subjected to blooming, forging, hot rolling or other hot-working (hereinafter sometimes called the “breakdown process”) to be given a slab shape that can be rolled with a hot-rolling mill.
Following the breakdown, the slab is made into a slab for hot rolling by further passage through a straightening process for enhancing flatness and treatments for removing surface scale and defects. This slab for hot rolling is processed into a strip coil (sheet) by heating to a prescribed temperature and hot rolling with a general purpose hot-rolling mill for steel or the like.
This hot-rolled strip coil may thereafter become a finished product in its form as annealed and/or descaled or become a finished product upon being further subjected to cold rolling or other cold working and annealing. In the descaling process after hot rolling, the surface scale and defects are removed, but the surface must be removed deeper in proportion as the surface defects are deeper, so that yield declines.
On the other hand, in the case of, for example, the electron beam melting method and plasma arc melting method, which use a hearth, the melting of the raw material is conducted with a controlled hearth independent of the mold, which increases mold shape freedom compared to vacuum arc melting, and as a result has the feature of enabling production of an ingot of rectangular cross-section.
In the case of producing flat material or strip coil from a rectangular ingot produced by the electron beam melting method or plasma arc melting method, it is possible in light of the ingot shape aspect to omit the aforesaid breakdown process, which leads to production cost reduction. Therefore, consideration is being given to technologies for producing rectangular ingots thin enough to be directly fed into a hot-rolling mill (sometimes called “as-cast slab”).
In producing such a thin titanium slab, a thinner rectangular mold than heretofore is required, and while fabrication of such a mold is not itself difficult, the casting surface properties and cast structure are considerably affected by the thickness and/or width of the mold and the casting conditions.
As for the casting surface properties of the as-cast slab, when pits/bumps, wrinkles or other deep defects are present, even if the surface of the as-cast slab is smoothed by machining or other treatment, any remaining bottom portions of the defects, even if slight, may become surface defects that become prominent after hot rolling. To avoid this, a process for treating and removing the surface of the as-cast slab to a considerable thickness becomes necessary.
Further, as shown in FIGS. 2 and 3, the as-cast structure is composed of coarse crystal grains of up to several tens of mm, and if this is directly hot rolled without being passed through a breakdown process, the coarse crystal grains cause uneven deformation that sometimes develop into large surface defects. As a result, yield is considerably degraded after hot rolling in the descaling process for removing surface defects, product inspection, and so on.
Therefore, with a titanium material, when the breakdown process is omitted, post-hot-rolling surface defects must be minimized as much as possible. Methods for smoothing the slab casting surface have been proposed to resolve this issue.
As technologies for improving the casting surface have been disclosed a method of extracting a titanium slab produced with an electron beam melting furnace from the mold and immediately feeding it to a surface shaping roll to smooth the cast slab surface (Patent Document 1) and a method of improving the casting surface of a cast slab by directing an electron beam onto the surface of a titanium slab extracted from a mold that is a component of an electron beam melting furnace to melt a surface layer portion and then feeding it to a surface shaping roll to produce a slab (Patent Document 2).
Even if the casting surface of a titanium slab produced with an electron beam melting furnace is smoothed by means like in Patent Document 1 or Patent Document 2, as pointed out above, defects often occur on the hot-rolled flat material owing to the cast structure of the original titanium slab.
In addition, Patent Document 1 and Patent Document 2 require an electron gun for titanium slab heating to be separately provided at the surface shaping roll or inside the electron beam melting furnace following extraction from the mold, so that an issue remains from the cost aspect.
As a melting method other than the electron beam melting method, the vacuum plasma melting furnace is sometime used. Non-patent Document 1 and Non-patent Document 2 disclose technologies for directly hot rolling a titanium slab produced with a vacuum plasma melting furnace into a strip coil (sheet).
In the technologies disclosed in Non-patent Document 1 and Non-patent Document 2, the melting rate is 5.5 kg/min, and because of the cross-sectional shape of the mold, the slab extraction rate is very slow, at about 0.38 cm/min, and the coil after hot rolling is passed through a grinding line (hereinafter sometimes called a “CG line”).
Because of this, the post-hot-rolled coil has surface defects and it is thought that the defects are removed by the CG line. Thus, like the titanium slab produced with an electron beam melting furnace, a problem exists in that defects occur on the surface of the hot-rolled flat material.
Further, the vacuum plasma melting method (plasma arc) does not permit deflection as with the electron beam for electron beam melting, making it awkward at regulating the irradiation site in the melting furnace and the balance of the amount of heat supplied, so that control of the casting surface and/or cast structure is not easy.
Thus, in the titanium slab produced with an electron beam melting furnace or the like, surface defects are produced by the hot rolling of the strip coil (flat material) owing to both the remaining casting surface defects and the cast structure, and a technology for producing a titanium slab suitable for hot rolling is therefore desired.